Method for applying a high-temperature bond coat on a metal substrate, and related compositions and articles

ABSTRACT

A method for applying a bond coat on a metal-based substrate is described. A slurry which contains braze material and a volatile component is deposited on the substrate. The slurry can also include bond coat material. Alternatively, the bond coat material can be applied afterward, in solid form or in the form of a second slurry. The slurry and bond coat are then dried and fused to the substrate. A repair technique using this slurry is also described, along with related compositions and articles.

This application is a division of application Ser. No. 10/256,602, filedSep. 30, 2002, now abandoned, which is a division of application Ser.No. 09/614,248 filed on Jul. 12, 2000, now U.S. Pat. No. 6,497,758, eachof which is hereby incorporated by reference in its entirety.

This invention was made with government support under Contract No.DEFC21-95-MC31176 awarded by the Department of Energy. The governmentmay have certain rights to the invention.

BACKGROUND OF THE INVENTION

The invention disclosed herein generally relates to bond coatings andthermal barrier coatings applied to metals. The metals are frequentlyportions of components used in turbine engines. The invention alsorelates to processes for depositing such coatings.

Components formed of specialty materials like superalloys are used invarious industrial applications, under a diverse set of operatingconditions. In many cases, the components are provided with coatingswhich impart several characteristics, such as corrosion resistance, heatresistance, oxidation resistance, and wear resistance. As an example,the various components of turbine engines, which typically can withstandin-service temperatures in the range of about 1100° C.–1150° C., areoften coated with thermal barrier coatings (TBC's), to effectivelyincrease the temperature at which they can operate.

Most TBC's are ceramic-based, e.g., based on a material like zirconia(zirconium oxide), which is usually chemically stabilized with anothermaterial such as yttria. For a jet engine, the coatings are applied tovarious superalloy surfaces, such as turbine blades and vanes, combustorliners, and combustor nozzles. Usually, the TBC ceramics are applied toan intervening bond coating (sometimes referred to as a “bond layer” or“bond coat”) which has been applied directly to the surface of the metalpart. The bond coat is often critical for improving the adhesion betweenthe metal substrate and the TBC.

The effectiveness of a TBC is often measured by the number of thermalcycles it can withstand before it delaminates from the substrate whichit is protecting. In general, coating effectiveness decreases as theexposure temperature is increased. The failure of a TBC is oftenattributed to weaknesses or defects related in some way to the bondcoating, e.g., the microstructure of the bond coating. TBC failure canalso result from deficiencies at the bond coating-substrate interface orthe bond coating-TBC interface.

The microstructure of the bond coating is often determined by its methodof deposition. The deposition technique is in turn determined in part bythe requirements for the overlying protective coating. For example, manyTBC's usually require a very rough bond coat surface (e.g., a root meansquare roughness (R_(a)) value of greater than about 200 micro-inches),for effective adhesion to the substrate. An air plasma spray (APS)technique is often used to provide such a surface.

There continues to be a need in the art for bond coatings which providevery good adhesion between the substrate and a subsequently-applied TBC,e.g., bond coatings with a relatively rough surface. Furthermore, newprocesses for applying and curing such coatings in regions of asubstrate which are somewhat inaccessible are also of great interest.(Conventional thermal spray equipment is sometimes too large andcumbersome for such regions). Moreover, the entire TBC system—bondcoating with the TBC itself—should exhibit good integrity duringexposure to high temperatures and frequent thermal cycles. Such a systemshould be effective in protecting components used in high performanceapplications, e.g., superalloy parts exposed to high temperatures andfrequent thermal cycles.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a method forapplying a bond coat on a metal-based substrate, comprising thefollowing steps:

a) applying a slurry which comprises braze material to the substrate,wherein the slurry also contains a volatile component;

b) applying bond coat material to the substrate;

c) drying the slurry and bond coat material under conditions sufficientto remove at least a portion of the volatile component; and

d) fusing the braze material and bond coat material to the substrate.

The braze material is usually based on nickel, cobalt, or iron. The bondcoat material is often an “MCrAlX” material or a metal carbide, asdiscussed below.

There are a variety of methods for applying the bond coat according tothis invention. One method calls for combining the bond coat materialand the braze material with a solvent and one or more additives, asdescribed below. The combined slurry mixture can then be deposited onthe substrate by various techniques, such as flow-coating, brushing, orspraying. As an alternative, the slurry applied in step (a) includes thebraze material but not the bond coat material, and is substantiallydried to form a green layer. An adhesive can be applied to the greenlayer, and the bond coat particles can then be applied to the adhesive,prior to the fusing step. As another alternative, two separate slurriescan be employed—one containing the braze material, and the othercontaining the bond coat material. Each slurry can contain the additivesdescribed below. In this embodiment, the braze slurry is usually appliedfirst, followed by the application of the bond coat slurry. The slurriescan then be dried and fused to the substrate. An overcoat can optionallybe applied over the bond coat. The overcoat is usually a conventionalthermal barrier coating, e.g., one based on zirconium. Alternatively,the overcoat can be of another type, such as a metal carbide-based wearcoating.

A method for replacing a bond coat applied over a metal-based substrateis also described below. The following steps are usually included inthis method:

(i) removing the existing bond coat from a selected area on thesubstrate;

(ii) applying a slurry which comprises braze material to the selectedarea, wherein the slurry also contains a volatile component;

(iii) applying additional bond coat material to the selected area; and

(iv) fusing the braze material and bond coat material to the selectedarea.

This technique can be part of the overall process for repairing a wornor damaged TBC system.

Another embodiment of this invention is directed to a unique slurrycomposition, containing a braze material and a bond coat material, alongwith other conventional slurry ingredients, such as a solvent. Asdiscussed elsewhere, the braze material is usually nickel, cobalt, iron,a precious metal, or some mixture containing one of those components.The bond coat material is usually of the MCrAlX-type (discussed below),or can be a metal carbide or other type of material. The slurrycomposition is very useful in the formation of a TBC system.

An article constitutes another embodiment of this invention. Itcomprises:

(a) a metal-based substrate, and

(b) a volatile-containing slurry on the substrate, comprising brazematerial and bond coat material (e.g. roughness-producing bond coatparticles).

The substrate is often a superalloy, and the braze material and bondcoat materials are as described below. When the volatile component inthe slurry has been substantially removed, a green coating remains,which is fused to the substrate, e.g., by brazing. As fused, the brazematerial forms a continuous matrix phase in which the bond coatparticles are embedded.

Other features and advantages of the present invention will be moreapparent from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional photomicrograph of a comparative coatingsystem which includes a bond coat and a TBC, both applied byconventional methods.

FIG. 2 is a cross-sectional photomicrograph of a bond coat/TBC coatingsystem, in which the bond coat was applied by a slurry techniqueaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The braze material used in this invention can be formed of an alloycomposition which is known in the art and commercially available. Twoclasses of braze compositions are frequently used: standard liquidbrazes and activated diffusion brazes. Very often (but not always), thebraze alloy has a composition similar to that of the substrate. Forexample, if the substrate is a nickel-base superalloy, the braze alloyusually contains at least about 40% by weight nickel, along with variousother elements, like chromium, aluminum, and yttrium. (Nickel-containingbraze or cobalt-containing braze alloys are usually used withcobalt-base superalloys). The braze alloy composition also typicallycontains one or more components for lowering its melting point. Examplesof melting point suppressants for nickel-base and cobalt-base brazealloy compositions are silicon, boron, and phosphorous. Silicon orboron, or combinations thereof, are often preferred. The braze alloycomposition may also contain other additives known in the art, e.g.,fluxing agents. The average particle size of the braze alloy is usuallyin the range of about 20 microns to about 150 microns, and morepreferably, in the range of about 40 microns to about 80 microns.

Illustrative nickel-base and cobalt-base braze alloy compositions aredescribed in the commonly-assigned U.S. patent application Ser. No.09/444,737 (W. Hasz), filed on Nov. 23, 1999, and incorporated herein byreference. Some preferred nickel-base braze alloy compositions for thepresent invention comprise about 5 wt % to about 15 wt % silicon orboron; and about 15 wt % to about 25 wt % chromium, with the balancebeing nickel. Silicon is sometimes preferred over boron. Mixtures ofsilicon and boron are also possible.

Other types of braze alloys may be used, e.g., precious metalcompositions containing silver, gold, platinum, and/or palladium, incombination with other metals, such as copper, manganese, nickel,chromium, silicon, and boron. Mixtures which include at least one of thebraze alloy elements are also possible. Many of the metal brazecompositions are available from Praxair Surface Technologies, Inc.

As mentioned above, the braze material is utilized in the form of aslurry. The slurry usually contains at least one binder and a solvent.Selection of the solvent depends on various factors, such as itscapacity for solubilizing the binder and dispersing the braze powder; aswell as the manner in which the slurry will be applied to the substrate.The braze material can usually be dispersed in either an aqueous ororganic solvent. Examples include water, ethanol or other alcohols;ketones, nitrile solvents (e.g., acetonitrile); ketone-type solventslike acetone; aromatic solvents like toluene, xylene, or xylenol; andcompatible mixtures thereof. Sometimes, a two-solvent system ispreferred, in which one solvent flash-evaporates, while the otherevaporates more slowly and provides leveling properties. (As used inthis disclosure, the term “volatile component” generally refers to thesolvent (or multiple solvents) used in the slurry. It should also beunderstood that the binders and other ingredients in the slurry willalso volatilize, or will decompose, as the temperature is raised, e.g.,as fusing temperatures are approached).

A variety of binder materials may be used in the slurry, e.g.,water-based organic materials such as polyethylene oxide and variousacrylics, or solvent-based binders. The slurry may also contain variousother additives, such as dispersants, wetting agents, deflocculants,stabilizers, anti-settling agents, thickening agents, plasticizers,emollients, lubricants, surfactants, anti-foam agents, and curingmodifiers. In general, the additives are each used at a level in therange of about 0.01% by weight to about 10% by weight, based on theweight of the entire slurry composition. Those skilled in the art candetermine the most effective level for any of the additives, withoutundue effort.

Conventional details related to the mixing of the slurry are describedin various references, such as U.S. Pat. No. 4,325,754, which isincorporated herein by reference. (Slurry compositions are alsocommercially available). A variety of techniques can be used to applythe slurry to the substrate. Examples include slip-casting, brushing,painting, dipping, flow-coating, roll-coating, spin coating, andspraying. Various texts are instructive in this regard, e.g., theKirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Vol. 5,pp. 606–619; as well as the Technology of Paints, Varnishes andLacquers, Edited by C. Martens, Reinhold Book Corporation, 1968. U.S.patent application Ser. No. 09/378,956 (D. Sangeeta et al, filed Aug.23, 1999 and commonly-assigned) also describes some aspects of slurrytechnology, and is incorporated herein by reference.

Various types of bond coat materials can be used in the presentinvention. Most are well-known in the art. “High-temperature” bond coatsare often (but not always) preferred. These are bond coats used inapplications where the substrate is exposed to service temperatures ofat least about 500° C., and more often, at least about 900° C. Veryoften, the bond coat material is of the MCrAlX type, where “M” can bevarious metals or combinations of metals, such as Fe, Ni, or Co; andwhere “X” is selected from the group consisting of Y, Ta, Si, Hf, Ti,Zr, B, C, and combinations thereof. (“X” is usually yttrium). Some ofthe preferred alloys of this type have a broad composition (in weightpercent) of about 17% to about 23% chromium; about 4% to about 13%aluminum; and about 0.1% to about 2% yttrium; with M constituting thebalance. In some embodiments, M is a mixture of nickel and cobalt,wherein the ratio of nickel to cobalt is in the range of about 10:90 toabout 90:10, by weight.

As alluded to earlier, other types of bond coat materials can be used.Non-limiting examples include aluminide, platinum-aluminide;nickel-aluminide; platinum-nickel-aluminide, and mixtures thereof.Moreover, mixtures of MCrAlX-type materials with metals such aszirconium or hafnium may also be used. Those skilled in the art will beable to select the most appropriate bond coat material, based on enduse, cost, processing method, and other considerations.

The size of the bond coat particles may vary somewhat, and is related inpart to the desired roughness for the bond coat. Usually, the bond coatparticles have an average particle size of at least about 45 microns. Inthose instances in which a subsequent TBC is to be applied by air plasmaspray (which often requires a rough, underlying surface), the bond coatparticles usually have an average particle size of at least about 150microns. In some preferred embodiments, the bond coat particles have asize in the range of about 150 microns to about 300 microns. Largerparticle sizes may be used in some cases, e.g., when greater roughnessis desired. Sometimes, these particles are referred to herein as the“primary bond coat particles”, which provide the conventional type ofroughness (R_(a)) discussed below.

In preferred embodiments, especially in situations where an airplasma-sprayed TBC is to be applied, the bond coat particles have aparticular shape. The shape is sufficient to produce micro-roughness inthe bond coat after the material has been fused to the substrate withthe braze. The micro-roughness is distinct from, and in addition to, theroughness (R_(a)) provided by the primary bond coat particles. (Theconventional roughness is usually measured by surface profilometry.)Micro-roughness is a fine-scale roughness and back-folding on theprimary particle. All embodiments of the present invention provide verygood adhesion for a subsequently-applied TBC. However, the presence ofthe micro-roughness greatly increases that adhesion in many instances,during the service life of the TBC.

There are several ways of obtaining micro-roughness, and they usuallyinvolve the use of bond coat powder which is commercially available andknown to provide such an effect. The micro-roughness may be in the formof smaller spheres of the bond coat material (e.g., having a size in therange of about 5% to about 50% of the primary particle diameter) whichare bonded to the larger, primary particles.

Alternatively, the micro-roughness may be in the form of an irregular orrough surface on the primary particles. In this instance, the surface ofthe primary particles is convoluted and somewhat jagged, with undercutsections which appear to back-fold in some areas. Such a particlesurface has an appearance similar to that resulting when one would tearan English muffin in half. Powder particles having these characteristics(e.g., with an MCrAlX-type composition) are commercially available.

In one embodiment of this invention, the slurry also contains the bondcoat material, so that the braze material and the bond coat material areapplied to the substrate simultaneously. Any convenient technique forcombining the braze and bond coat materials in a single slurry may beemployed, e.g., mechanical mixers. In addition to following generalsafety procedures, care should be taken to keep each of the metalcomponents well-dispersed in the slurry. At least one aqueous or organicsolvent is used for the slurry. Choice of a particular solvent orsolvent mixture for this embodiment will depend in part on the solvent'scompatibility with both the braze and bond coat materials, as well aswith any melting point suppressant which may be present. The solventsshould also be capable of maintaining the solid components substantiallydispersed. Moreover, the additives (mentioned above) which are used inthe slurry should be compatible with each other, and with the othercomponents in the slurry.

The slurry is usually deposited on the substrate as a single layer.However, in some instances, it may be desirable to deposit the slurry inthe form of at least two “sub-layers”, i.e., in at least twoapplications. For example, each sub-layer may include the samecomposition, but the size of the bond layer particles may be varied.Smaller particles may be used in the sub-layer closest to the substrate,for increased coating density. Larger particles may be used in one ormore upper sub-layers, to provide a desired amount of roughness. (A heattreatment could be applied after the application of each sub-layer).

As another alternative, the composition of two or more sub-layers can bevaried, to provide different properties at different depths of the bondcoat. As an example, a first sub-layer could contain a standardNiCrAlY-type bond coat material, along with the braze alloy. A secondsub-layer applied over the first sub-layer could contain braze alloy,along with a different bond coat material, e.g., an MCrAlX-type bondcoat material in which M is a mixture of nickel and cobalt. The secondsub-layer, which is closer to the atmosphere during service, shouldprovide greater corrosion resistance than the standard NiCrAlY materialin some environments.

In a similar fashion, the composition of two or more sub-layers could bevaried to adjust the degree to which oxidation occurs, e.g., oxidationat the bond coat-substrate interface, as discussed in the examples.Moreover, the composition of the bond coat could be graded or layered,(e.g., by a metering system), so that the change in particularconstituents is gradually made as the slurry component is applied overthe substrate.

After the slurry mixture has been deposited, at least a portion of thevolatile material contained therein is removed. This step is sometimesreferred to as an “evaporation step” or “evaporation stage”, and resultsin a substantially-devolatilized (solvent-free) coating, i.e., a “green”coating. Any convenient drying technique can be used to remove thevolatile component. Drying may include air- or vacuum-drying at roomtemperature. In some instances, it may be desirable to heat the slurrymixture to accelerate drying.

The green coating, which contains the braze material and the bond coatmaterial, is then fused to the substrate. The fusing step can be carriedout by various techniques. Very often, it is a brazing step, and issimilar to any conventional brazing operation. (As used herein,“brazing” is generally meant to include any method of joining metalsthat involves the use of a filler metal or alloy.) One exemplaryreference for details regarding brazing is the text Modem Metalworking,by J. R. Walker, The Goodheart-Willcox Co., Inc., 1965, pp. 29–1 to30–24. Those of ordinary skill in the art are familiar with otherdetails regarding brazing. Brazing temperatures depend in part on thetype of braze alloy used, and are typically in the range of about 525°C. to about 1650° C. In the case of nickel-based braze alloys, brazetemperatures are usually in the range of about 800° C. to about 1260° C.When possible, brazing is often carried out in a vacuum furnace. Theamount of vacuum will depend in part on the composition of the brazealloy. Usually, the vacuum will be in the range of about 10⁻¹ torr toabout

10⁻⁸ torr. Furnace brazing also removes any volatile materials (e.g.,the binder) remaining in the green coating. Volatile content can bedetermined by a variety of techniques, such as differential thermalanalysis (DTA) and thermal gravimetric analysis (TGA).

In some cases, the slurry may have to be applied to an area which doesnot lend itself to the use of a furnace. As an example, the componentitself may be too large for insertion into a furnace. In such a case,alternatives are possible. For example, a torch or other localizedheating means can be used. These techniques are known in the art andbriefly described in the commonly-assigned patent application of W.Hasz, Ser. No. 09/444,737, mentioned above and incorporated herein byreference.

In an alternative embodiment, the slurry contains the braze material andany necessary additives, but does not contain the bond coat material. Inthis instance, the slurry is substantially dried after being applied, toform a green layer. Any convenient drying technique can be used, likethose described above, e.g., air drying, prior to or following anoptional heat treatment to increase the evaporation of the volatilecomponent.

The bond coat material—usually in the form of dry powder particles—isthen applied over the green layer. Usually, an adhesive is applied to asurface of the green layer, prior to the application of the bond coatpowder. A variety of adhesives can be used, as long as they are capableof completely volatilizing during the subsequent fusing step. Some ofthe suitable adhesives are described, for example, in The CondensedChemical Dictionary, 10th Edition, B. Hawley, Van Nostrand ReinholdCompany Inc., 1981, pp. 20–21, which is incorporated herein byreference. Illustrative examples of adhesives include polyethylene oxideand acrylic materials. Commercial examples of braze adhesives include“4B Braze Binder™”, available from Cotronics Corporation. The adhesivecan be applied by various techniques. For example, liquid-like adhesivescan be sprayed or coated onto the surface. A thin mat or film withdouble-sided adhesion could alternatively be used, e.g., 3M Company's467™ Adhesive Tape.

The bond coat powder can then be randomly applied over the adhesive by avariety of techniques, e.g., sprinkling, pouring, blowing,roll-depositing, and the like. After the deposition, the excess powderis removed from the substrate (e.g., by shaking or by being blown off),leaving substantially a single layer of bond coat particles. Asdescribed previously, the size of the particles depends in large part onthe degree of roughness required for the bond coat. The green coating(to which the braze material is attached) is then fused to thesubstrate, as described above. The resulting coating system issubstantially identical to that formed in the first embodiment.

In another alternative embodiment, the bond coat material can be used inthe form of a second slurry, i.e., separate from the slurry whichcontains the braze material. The second slurry would be formed with atleast one solvent, i.e., a solvent compatible with the particular bondcoat composition. The slurry would also contain one or more of the otheradditives described previously, e.g., binders, dispersants, and thelike. Moreover, the slurry can be applied over the first slurry by anyof the techniques described above, such as spraying. In preferredembodiments, some or all of the volatile component in the first slurryis removed before the application of the second slurry, to avoidbubbling. Removal of the volatiles is usually carried out by heating, asdescribed above. Volatiles are then removed from the second slurry in asame or similar manner, prior to the fusing step. The resulting coatingsystem is substantially identical to that formed in the otherembodiments.

As another alternative, the bond coat can be in the form of a secondslurry which is then pre-mixed with the first slurry. The resultingpre-mixture can be applied to the substrate, prior to removal of thevolatile component. Fusing is then carried out in the manner describedpreviously.

In some embodiments of the present invention, an overcoat is appliedover the bond coat, after the bond coat material has been fused to thesubstrate with the braze material. The overcoat is usually a thermalbarrier coating, but it could be any type of coating which providesenvironmental protection, i.e., protection of the substrate from theadverse effects of oxidation, corrosion, or chemical attack. Theovercoat could also be a wear-resistance coating. Moreover, the overcoatis usually ceramic, but could alternatively be metallic.

Ceramic thermal barrier coatings are often (but not always)zirconia-based. As used herein, “zirconia-based” embraces ceramicmaterials which contain at least about 50% zirconia, by weight. Zirconiais a well-known compound for barrier coatings. Its use is described, forexample, in Kirk-Othmer's Encyclopedia of Chemical Technology, 3rdEdition, V. 24, pp. 882–883 (1984). In preferred embodiments, thezirconia is chemically stabilized by being blended with a material suchas yttrium oxide, calcium oxide, magnesium oxide, cerium oxide, scandiumoxide, or mixtures of any of those materials. In one specific example,zirconia can be blended with about 1% by weight to about 20% by weightyttrium oxide (based on their combined weight), and preferably, fromabout 3%–10% yttrium oxide.

Various techniques can be used to apply the ceramic coating.Non-limiting examples include a thermal spray technique such as APS;physical vapor deposition (PVD); or electron beam physical vapordeposition (EB-PVD). Those of ordinary skill in the art are familiarwith the details regarding each of these deposition techniques. Relatedreferences include Kirk-Othmer's Encyclopedia of Chemical Technology,3rd Edition, Vol. 15, (1981) and Vol. 20 (1982); Ullmann's Encyclopediaof Industrial Chemistry, Fifth Edition; Volume A6, VCH Publisher (1986);Scientific American, H. Herman, September 1988; and U.S. Pat. No.5,384,200. Ceramic slurry techniques or sol gel techniques can also beused to apply the ceramic coating.

Examples of other types of materials for the overcoat includewear-resistant coatings, e.g., carbide coatings such as chromium carbideand tungsten carbide, and those formed fromcobalt-molybdenum-chromium-silicon. Other types of material could beused as well, e.g., alumina, mullite, zircon, and glassy-type materialssuch as strontium-calcium-zirconate glass. Those of ordinary skill inthe art will be able to select the most appropriate material for a givenend use application. Methods for preparing and applying such materialsare those described above for the zirconia TBC's, or consist of othertechniques well-known in the art. Moreover, some of the overcoats can beprepared and applied in the form of a slurry over the bond coat, asmentioned above. Slurry-based overcoats are also described in thecommonly-assigned U.S. patent application of D. Sangeeta, Ser. No.09/557,393, filed on Apr. 24, 2000 and incorporated herein by reference.For example, the Summary of the Invention and other sections of thatpatent application are instructive.

Another embodiment of this invention is directed to a method forreplacing a bond coat previously applied to a metal-based substrate. Thereplacement of the bond coat is often a part of the overall process ofrepairing a worn or damaged TBC. Careful repair of a TBC “system” (bondcoat and TBC) is critical in preventing degradation of the substrate. Inthe case of a turbine engine component, for example, it may be necessaryto repair the coating while the turbine is in service, i.e., after itsdelivery from the manufacturing site. The process disclosed hereinprovides a means for rapidly repairing or replacing selected areas of anexisting TBC system, without having to completely remove the coatingsfrom the entire part. The process is especially useful for repairingcoatings which are situated in areas not easily accessible to otherrepair techniques.

The steps for replacing the bond coat usually comprise:

(i) removing the existing bond coat (and a worn or damaged overcoat, ifpresent) from a selected area on the substrate;

(ii) applying a slurry which comprises braze material to the selectedarea, wherein the slurry also contains a volatile component;

(iii) applying additional bond coat material to the selected area; and

(iv) fusing the braze material and bond coat material to the selectedarea.

As described previously, a single slurry can be used, containing boththe braze material and the bond coat material. Alternatively, twoseparate slurries can be used. As another alternative, a braze slurrycan be applied and then dried, followed by the application of anadhesive layer. Bond coat material can then be applied to the adhesivelayer.

The slurry and bond coat material can be air-dried between steps (iii)and (iv). Heating means, such as an IR lamp, may be used to accelerateremoval of the volatile component. The fusing step is often carried outby using a torch or other portable heating apparatus.

In the case of a turbine engine component which includes the coatingbeing repaired, the heat evolved during engine operation may besufficient to remove the volatile component and carry out fusing step(iv). This means of heating and curing could in fact be postponed untila slurry-based overcoat has been applied, as described below.

An overcoat can then be applied over the bond coat, in those instancesin which an overcoat is being replaced. Usually, the overcoat (e.g., aTBC) will be applied by a thermal spray process in a repair setting.Plasma spraying is one convenient technique. However, the overcoat canalternatively be applied over the bond coat in the form of a slurry, asdiscussed above (application Ser. No. 09/557,393 of Sangeeta). Asmentioned earlier, turbine engine operation temperatures may besufficient to remove all volatile material, fuse the braze material andbond coat to the substrate; and cure the overcoat, all in a single step.

Another embodiment of this invention is a slurry composition whichcomprises braze material and bond coat material. Such a slurry is usefulfor applying a bond coat, as described above. Standard liquid brazes oractivated diffusion brazes can be used in the slurry. When the slurry isbeing applied to a nickel-base superalloy, the braze alloy usuallycontains at least about 40% by weight nickel, along with various otherelements, like chromium, aluminum, and yttrium. The average particlesize of the braze alloy is usually in the range of about 20 microns toabout 150 microns, as mentioned above.

The bond coat material in the slurry is usually of the MCrAlX type, asdescribed previously. The size of the bond coat particles may varysomewhat. They often have an average particle size of at least about 45microns.

The choice of solvent for the slurry will depend in part on the solidcomponents contained therein, and on the manner in which it will beapplied to the substrate. Exemplary solvents are described above, alongwith binder materials and a variety of other additives, e.g.,dispersants, wetting agents, and stabilizers. The level of brazematerial and bond coat in the slurry will depend on various factors,such as the desired thickness of the bond coat; the solubility anddispersibility of the bond coat and braze materials in the solvent orsolvent mixture; and the manner in which the slurry will be applied.Usually, the slurry comprises about 20% by weight to about 50% by weightbraze material, and about 50% by weight to about 80% by weight bond coatmaterial, based on total slurry weight. The slurry typically containsabout 10% or less by weight solvent, and about 10% or less by weightbinder.

Still another embodiment of this invention is directed to an article,comprising:

(a) a metal-based substrate, e.g., one formed of a superalloy; and

(b) a volatile-containing slurry on the substrate, comprising brazematerial and bond coat material. The particular components which may befound in such a slurry have been described previously. When the volatilecomponent in the slurry has been substantially removed, a green layerremains. The green layer is then fused to the substrate, e.g., by abrazing technique. In preferred embodiments, the braze material forms acontinuous matrix phase in which the bond coat particles are embedded.The size of the bond coat particles can be selected to cause them toprotrude beyond the matrix phase. In that instance, they serve as arelatively rough surface, e.g., one having an R_(a) of greater thanabout 200 micro-inches, and preferably, greater than about 300micro-inches. Such a surface provides excellent adhesion to asubsequently-applied ceramic layer. An article containing such a layer(e.g., a zirconia-based TBC) is also within the scope of this invention.

In order that those skilled in the art may better understand theinvention, the following examples are provided by way of illustration,and not by way of limitation.

EXAMPLE 1

Sample A was provided for the purpose of comparison, and represents atypical TBC system. The substrate was a coupon made from a nickel-basesuperalloy. The coupon was grit-blasted and ultrasonically cleaned. ANiCrAlY-type bond coat was then air plasma-sprayed (APS) onto thesubstrate surface. The nominal bond coat composition was as follows: 68wt % Ni, 22 wt % Cr, 9 wt % Al, and 1 wt % Y. The thickness of the bondcoat was in the range of about 5–8 mils (about 127–203 microns). It hadan average roughness R_(a) of about 500 to about 900 micro-inches. A TBC(thermal barrier coating: yttria-stabilized zirconia, with 8 wt % byweight yttria) was then air plasma-sprayed over the bond coat. Thethickness of the TBC was in the range of about 10–12 mils (about 254–305microns).

Sample B represented an embodiment of the present invention. A slurrywas first prepared by adding the following components to acetone, underagitation:

(a) coarse NiCrAlY-type bond coat powder, having an approximatecomposition as follows: 68 wt % Ni, 22 wt % Cr, 9 wt % Al, and 1 wt % Y.The powder had an average particle size of −30 +100 mesh, i.e., 150–600microns.

(b) High-temperature braze powder, commercially available as Amdry® 100,having the following approximate composition: 10% by weight silicon; 19%by weight chromium, base nickel. The powder had an average particle sizeof about-100 mesh, i.e., less than about 150 microns.

(c) Nicrobraz® 300 binder (ethyl methacrylate in trichloroethane;available from Wall Colmonoy, Inc., Madison Heights, Mich.)

The metal powders were dry-mixed (50 wt % of component (a) and 50 wt %of component (b)). Components (c) and (d) were then added and mixed (10wt % of each to the total slurry weight).

The slurry was applied (by brushing) to the same type of superalloycoupon used for sample A. The wet thickness of the slurry was about 5mils (127 microns). The slurry was then allowed to air-dry for about 12hours, during which at least about 15 wt % of the volatile material wasremoved. The resulting green coating was then heated in a vacuum furnaceat a brazing temperature of about 1093–1204° C. (2000–2200° F.), forabout 1 hour. A dense, rough bond coat was produced, having an R_(a) ofabout 25 microns (about 984 micro-inches). The same type of thermalbarrier coating (zirconia-based) used in sample A was then airplasma-sprayed over the bond coat.

FIG. 1 is a cross-sectional photomicrograph of the coating system forsample A. Region I is the substrate. Region II is an oxide region whichhas begun to form between the substrate and the bond coat, as a resultof thermal testing. Region III is the bond coat itself, exhibiting thetypical overlapping of bond coat material “splats” which result from APSdeposition. Region IV is the TBC.

The overall coating system of sample A exhibits good integrity for someend uses, and for a projected service life. However, Region II resultsfrom accelerated oxidation at the bond coat-substrate interface, towardthe end of a simulated service life. The oxidation will eventuallyresult in coating failure, by causing most or all of the TBC and bondcoat to become detached from the substrate.

FIG. 2 is a cross-sectional photomicrograph of the coating system forsample B, prepared according to an embodiment of the present invention.The coating system was subjected to the same amount of thermal testingas sample A. Region V is the substrate. Region VI is the slurry layerapplied and brazed over the substrate. Region VII is the TBC. Theabsence of the oxidation region seen in FIG. 1 indicates thataccelerated oxidation at the bond coat-substrate interface has not takenplace.

The type of thermal testing carried out for each sample was FurnaceCycle Testing (FCT). One cycle represented 45 minutes at 2000° F. (1093°C.). The process continued for 300 cycles for each sample. The resultsdemonstrated that the present invention (sample B) had a furnace cyclelife which was about 3 times greater than that of the comparative baseline sample (sample A).

EXAMPLE 2

In this example, the same type of substrate was used, to prepare sampleC. A slurry was prepared, containing 80 wt % of the Amdry® 100high-temperature braze powder, along with 10 wt % water and 10 wt %polyethylene oxide binder. The slurry was applied to the substrate, to awet thickness of about 5 mils (127 microns). The slurry was then allowedto dry for about 14–16 hours. About 15 wt % of the volatile material wasremoved in this drying step, leaving a green layer. A layer of brazecontact adhesive (Nicrobraz™ 300) was then applied over the green layer.

The coarse NiCrAlY-type bond coat powder used in Example 1 (sample B)was then sprinkled onto the contact adhesive, to produce a mono-layer ofcoarse powder. The excess bond coat powder was blown off. The coupon wasthen heated in a vacuum furnace at a brazing temperature of about1093–1204° C. (2000–2200° F.), for about 0.25–2 hours, to produce adense, rough bond coat (R_(a) of about 25 microns/984 micro-inches). Thesame type of thermal barrier coating (zirconia-based) used in sample Awas then air plasma-sprayed over the bond coat.

FCT tests were carried out, as in Example 1. Sample C displayedapproximately the same properties (resistance-to-cracking anddelamination) as those of sample B. Moreover, sample C showed noindication of the accelerated oxidation at the bond coat-substrateinterface, i.e., as evidenced for sample A.

Preferred embodiments have been set forth for the purpose ofillustration. However, the foregoing description should not be deemed tobe a limitation on the boundaries of the invention. Accordingly, variousmodifications, adaptations, and alternatives may occur to one skilled inthe art without departing from the spirit and scope of the presentinvention.

All of the patents, articles, and texts mentioned above are incorporatedherein by reference.

1. A method for applying a bond coat on a metal-based substrate,comprising the following steps: a) applying a slurry which comprisesbraze material to the substrate, wherein the slurry also contains avolatile component, and wherein the braze material has an avengeparticle size in the range of about 20 microns to about 150 microns; b)applying bond coat material to the substrate, wherein the slurrycomprising the braze material further comprises the bond coat material,so that the braze material and the bond coat material are applied to thesubstrate simultaneously, wherein said bond coat material comprises atleast one material selected from the group consisting of aluminide;platinum-aluminide; nickel-aluminide; platinum-nickel-aluminide; analloy of the formula MCrAlX, where M is selected from the groupconsisting of Fe, Ni, Co, and mixtures of any of the foregoing and whereX is selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C,and combinations thereof; and any combination of the foregoing; c)drying the slurry and bond coat material to remove at least a portion ofthe volatile component; and d) fusing the braze material and bond coatmaterial to the substrate.
 2. The method of claim 1, wherein the brazematerial comprises at least one metal selected from the group consistingof nickel, cobalt, iron, a precious metal, and a mixture which includesat least one of the foregoing.
 3. The method of claim 2, wherein thebraze material comprises at least about 40% by weight nickel.
 4. Themethod of claim 2, wherein the braze material further comprises aconstituent which lowers the melting point of the braze alloy.
 5. Themethod of claim 4, wherein the constituent is silicon, boron, ormixtures thereof.
 6. The method of claim 1, wherein the slurry furthercomprises at least one additive selected from the group consisting ofbinders, stabilizers, thickening agents, dispersants, deflocculants,anti-settling agents, plasticizers, emollients, lubricants, surfactants,anti-foam agents, and curing modifiers.
 7. The method of claim 1,wherein the slurry is applied to the substrate by a technique selectedfrom the group consisting of slip-casting, brushing, painting, dipping,flow-coating, roll-coating, spin coating, and spraying.
 8. The method ofclaim 1, wherein the bond coat material has an average particle size ofat least about 45 microns.
 9. The method of claim 8, wherein the bondcoat material has an average particle size in the range of about 150microns to about 300 microns.
 10. The method of claim 1, wherein thevolatile component comprises at least one aqueous solvent or at leastone organic solvent, or mixtures thereof.
 11. The method of claim 1,wherein step (c) is carried out by air-drying.
 12. The method of claim1, wherein step (d) is carried out at a temperature in the range ofabout 525° C. to about 1650° C.
 13. The method of claim 1, wherein themetal-based substrate is a superalloy.
 14. The method of claim 13,wherein the superalloy is a nickel-base or cobalt-base material.
 15. Themethod of claim 1, wherein the bond coat has a root mean squareroughness (Ra) value of greater than about 200 micro-inches, after step(d).
 16. The method of claim 1, wherein the slurry is prepared bycombining the bond coat material and the braze material with a solventand at least one additive selected from the group consisting of binders,stabilizers, thickening agents, dispersants, deflocculants,anti-settling agents, plasticizers, emollients, lubricants, surfactants,anti-foam agents, and curing modifiers.
 17. The method of claim 1,wherein the braze material has an average particle size in the range ofabout 20 microns to about 150 microns, and the bond coat material has anaverage particle size of at least about 45 microns.
 18. The method ofclaim 1, wherein the bond coat is in the form of a second slurry, andthe second slurry is pre-mixed with the first slurry to form apre-mixture, said pre-mixture being applied to the substrate prior tostep (c).
 19. The method of claim 18, wherein the pre-mixture is appliedto the substrate by a technique selected from the group consisting ofslip-casting, brushing, painting, dipping, flow-coating, roll-coating,spin coating, and spraying.
 20. The method of claim 1, wherein anovercoat is applied over the bond coat after step (d).
 21. The method ofclaim 20, wherein the overcoat is a thermal barrier coating.
 22. Themethod of claim 21, wherein the thermal barrier coating iszirconia-based.
 23. The method of claim 21, wherein the thermal barriercoating is applied by a thermal spray technique.
 24. The method of claim23, wherein the thermal spray technique is a plasma spray process. 25.The method of claim 20, wherein the overcoat is a wear-resistantcoating.
 26. The method of claim 20, wherein the overcoat comprises amaterial selected from the group consisting of metal carbides; alumina,mullite, zircon, cobalt-molybdenum-chromium-silicon;strontium-calcium-zirconate glass; and mixtures thereof.
 27. A methodfor applying a metal aluminide- or MCrAlY-based bond coat on asuperalloy substrate, where M is nickel, cobalt, or a mixture thereof,comprising the following steps: (I) applying a slurry which comprises avolatile component and a mixture of braze material and bond coatmaterial to the substrate, wherein the braze material contains at leastabout 40% by weight nickel, and wherein the braze material has anaverage particle size in the range of about 40 microns to about 80microns and the bond coat material has an average particle size in therange of about 150 microns to about 300 microns; (II) drying the slurryunder conditions sufficient to remove at least a portion of the volatilecomponent, forming a green coating; and (III) brazing the green coatingto the substrate; wherein said bond coat material comprises at least onematerial selected from the group consisting of aluminide;platinum-aluminide; nickel-aluminide; platinum-nickel-aluminide; analloy of the formula MCrAlX, where M is selected from the groupconsisting of Fe, Ni, Co, and mixtures of any of the foregoing and whereX is selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C,and combinations thereof; and any combination of the foregoing.
 28. Themethod of claim 27, wherein a zirconia-based thermal barrier coating isapplied over the bond coat.
 29. A method for replacing a bond coatapplied over a metal-based substrate, comprising the following steps:(i) removing the existing bond coat from a selected area on thesubstrate; (ii) applying a slurry which comprises braze material to theselected area, wherein the slurry also contains a volatile component,and wherein the braze material has an average particle size in the rangeof about 20 microns to about 150 microns; (iii) applying additional bondcoat material to the selected area, wherein the slurry comprising thebraze material further comprises the bond coat material, so that thebraze material and the bond coat material are applied to the substratesimultaneously, wherein said bond coat material comprises at least onematerial selected from the group consisting of aluminide;platinum-aluminide; nickel-aluminide; platinum-nickel-aluminide; analloy of the formula MCralX, where M is selected from the groupconsisting of Fe, Ni, Co, and mixtures of any of the foregoing and whereX is selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C,and combinations thereof; and any combination of the foregoing; and (iv)fusing the braze material and bond coat material to the selected area.30. The method of claim 29, wherein the metal-based substrate is aportion of a turbine engine.